Stator winding control circuit for a brushless d.c. motor

ABSTRACT

A brushless d.c. motor includes a plurality of Hall elements 1-3 for detecting the relative positions of the rotor and stator poles, a comparator circuit 103 responsive to a bipolar input control signal A for generating an output signal B proportional to the absolute magnitude of signal A and an output signal D as a function of the polarity of signal A, a timing signal generator 104 for generating output signals E timed in relation to the Hall element outputs F and having a phase sequence in response to signal D, and a reversible current supply circuit 105 for driving a set of Y-connected stator windings 101 with currents C having a magnitude proportional to signal B and a phase sequence in accordance with signal E.

BACKGROUND OF THE INVENTION

This invention relates to a stator winding drive circuit for areversible, brushless d.c. motor, particularly adapted for use in amagnetic recording tape deck.

In conventional d.c. motors having a mechanical commutator, it is easilypossible to selectively produce a forward or a reverse torque by merelychanging the polarity of the commutator voltage. However, d.c. motors ofthis type have an inherent disadvantage in that considerable noise isproduced by the mechanical commutator, which adversely affects otherequipment around the motor.

In order to resolve this defect, a brushless d.c. motor has beendeveloped which employs Hall elements for detecting the rotor positionand semiconductor switching elements responsive to the detection of therotor position to switch the electric currents supplied to the windingsof the motor. That is, the combination of the Hall elements and thesemiconductor switching elements serves the same general function as themechanical commutator. Since this construction has no mechanicalcomponents, there is no possibility of noise generation.

Such a brushless d.c. motor has been successfully used. However, sincemost semiconductor switching elements cannot accomodate a current flowin both directions, it is very difficult to design a simple andeffective semiconductor switching circuit which can selectively providebidirectional currents to thereby produce reversible motor torques.

SUMMARY OF THE INVENTION

An object of the present invention is therefore to provide a simplifiedstator winding drive circuit for a brushless d.c. motor capable ofproducing a reversible torque, and hence reversible rotation of themotor, and capable of rapidly responding to rotation speed controlsignals.

These objects are achieved by employing a driving circuit for abrushless d.c. motor having a stator provided with multi-phase windingsand a rotor having magnetic poles, which comprises a position detectingmeans for providing an output signal in response to a predeterminedpositional relation between the stator and the rotor, a first means forproducing a first output in response to an absolute value of an inputcontrol signal and a second output in response to the polarity of theinput control signal, a second means for producing a plurality of timingsignals, the phase thereof being reversed according to the polarity ofthe input control signal, and a third means for sequentially driving thewindings of the stator with a current corresponding to the absolutevalue of the input control signal in response to the timing signals.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 is a block diagram of a driving circuit for a brushless d.c.motor according to the present invention;

FIG. 2 is a plot of a characteristic curve for explaining the operationof the present invention;

FIG 3. is a schematic circuit diagram of a preferred embodiment of thepresent invention; and

FIG. 4 shows a plurality of waveforms produced at various points of thecircuit in FIG 3.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Block 101 in FIG. 1 designates a stator winding unit, block 102 is aposition detector comprising Hall elements and resistors for producing asignal F in response to the relative position between the stator and therotor, block 103 is a circuit for producing a signal B in response tothe absolute value of an input control signal A obtained by comparing aspeed signal voltage of the motor with a reference voltage, and anoutput signal D in response to the polarity of the input control signalA, and block 104 is a timing signal generator which receives the outputsignal F from the position detector 102 and the polarity output D fromthe circuit 103. The timing signal generator 104 shapes the signal F toproduce a timing signal E, and switches the phase of the timing signalaccording to the polarity of the input control signal A such that whenthe polarity of signal A is positive the phase of the timing signal Ebecomes negative, and vice versa. Block 105 is a circuit responsive tosignal B from the circuit 103 and the timing signal E from the timingsignal generator 104 for selectively supplying an electric current C tothe respective stator windings 101.

When a positive or negative input control signal A is applied to thecircuit 103 the latter produces a voltage or current output Bcorresponding to the absolute value of the signal A. The output B isapplied to the circuit 105 in which it is switched by the timing signalE and amplified to produce a winding current C which is supplied to thestator windings 101.

On the other hand, the position detection signal F from the positiondetector 102 is shaped and amplified by the timing signal generator 104to obtain the timing signal E. The polarity of signal E is switchedaccording to the polarity of the input control signal A as mentionedearlier. Therefore, even when the output signal F from the positiondetector 102 represents that the stator and the rotor are in the sameposition, the phase of the timing signal E is changed according to thepolarity of the input control signal A so that the direction of thewinding current C is reversed, thus resulting in the direction of thetorque being reversed in accordance with the polarity of the inputcontrol signal.

According to this principle, it is possible to provide a brushless d.c.motor capable of producing a bidirectional torque, as shown in FIG. 2,which has a magnitude proportional to the absolute value of the inputcontrol signal A and a direction which is determined by the polarity ofthe control signal A.

FIG. 3 shows an embodiment of the present invention applied to a drivingcircuit for a three-phase brushless motor. In FIG. 3, the positiondetector 102 comprises Hall elements 1, 2, and 3 and resistors 4 - 9connected to the Hall elements for providing biasing voltages to thecurrent terminals thereof. The Hall elements detect the relativeposition of the stator and the rotor of the motor, and are mounted onthe stator assembly at spaced position around the periphery of therotor.

The timing signal generator 104 amplifies the output signal F, i.e., thesignals at the voltage terminals of the Hall elements 1 - 3, and shapesthe amplified voltages to obtain the timing signal E.

The timing signal generator 104 includes three pairs of differentialamplifiers 110, 111 and 112. Differential input terminals l-l', m-m' andn-n' are connected to the voltage terminals of the Hall elements 1, 2and 3, respectively, and are supplied with voltages e₁, e₂ and e₃ , FIG.4-(1), generated at such voltage terminals corresponding to the relativeposition of the rotor with respect to the stator. The abscissa in FIG. 4represents the relative position of the rotor to the stator. Thevoltages e₁, e₂ and e₃ are fed to the bases of transistor pairs 10-11,12-13 and 14-15 which form the input portions of the respective pairs ofdifferential amplifiers. The electric currents flowing through collectorterminals a, c and e of these transistors have waveforms as shown inFIGS. 4 - (2), (3) and (4), and the currents flowing through collectorterminals b, c and f have waveforms of equal amplitude to those shown inFIGS. 4 - (2), (3) and (4) but of opposite phase or polarity.Transistors 16 - 18 serve to stabilize the input currents.

The bases of transistors 19, 22, 23, 26, 27 and 30 are commonlyconnected to a line k, and the bases of transistors 20, 21, 24, 25, 28and 29 are commonly connected to a line j. When the voltage on line j islower than the voltage on line k transistors 19, 22, 23, 26, 27 and 30are turned on and transistors 20, 21, 24, 25, 28 and 29 are cutoff.Therefore, the current flowing through a load resistor 31 connected tothe collectors of transistors 19, 21, 24 and 26 is a combination of thecurrents in terminals a and d, and the waveform of the voltage producedat the output terminal g appears as shown in FIG. 4-(5). Similarly, thevoltage waveform at the output terminal h of the load resistor 32connected to the collectors of transistors 23, 25, 28 and 30, and thevoltage waveforms at the output terminal i of the load resistor 74connected to the collectors of transistors 20, 22, 27 and 29, appear asshown in FIGS. 4-(6) and (7), respectively.

On the other hand, when the potential on line j is higher than that online k transistors 19, 22, 23, 26, 27 and 30 are cutoff and transistors20, 21, 24, 25, 28 and 29 are turned on, and the waveforms obtained atterminals g, h and i appear as shown in FIGS. 4-(8), (9) and (10),respectively. Thus, it is clear that the polarities of the timing signalE, i.e. the voltages at terminals g, h, i, are reversed dependent on therelative potential relationship between lines j and k.

The voltages on lines j and k, which correspond to the signal D in FIG.1, are supplied from the circuit 103. The latter comprises,fundamentally, a pair of differential amplifiers having current outputs.One of the differential amplifiers comprises transistors 33-37, and theother comprises transistors 38-42. The input control signal A is appliedto the differential input terminals O and P. When the voltage applied toterminal O is higher than that applied to terminal P the collectorcurrent of transistor 34 becomes larger than that of transistor 33, andconsequently the potential at point q is reduced until diode 44,connected to an emitter follower transistor 43, is turned on.

On the other hand, the collector current of transistor 38 becomes largerthan that of transistor 39, and consequently the potential at point r isincreased to cut off the diode 45 and forward bias the diode 46 to feedbase current to a transistor 47 in the circuit 105. The constants of theelements in circuit 103 should be selected such that the potential atpoint r when diode 46 is turned on is higher than that at point q whendiode 44 is turned on. Since the constants are so selected that when thepotential at terminal O is higher than that at terminal P the potentialat point r becomes higher than that at point q, the emitter voltage oftransistor 48 whose base is connected to point r therefore becomeshigher than the emitter voltage of transistor 49 whose base is connectedto point q. Consequently, the potential on line k becomes higher thanthat on line j because the ratio of resistor 50 to resistor 51 is equalto that of resistor 52 to resistor 53. The potentials on lines j and kare applied to the timing signal generator 104. The timing signal E thusobtained is shown in FIGS. 4-(5), (6) and 7, as developed earlier.

Conversely, when the potential at terminal P is higher than that atterminal O, the phenomenon described above is reversed. That is, diode44 is turned off and diode 54 is turned on. Therefore, base current isfed to transistor 47 in circuit 105 and the potential on line j becomeshigher than that on line k, resulting in timing signals on lines g, hand i as shown in FIGS. 4-(8), (9) and (10).

In this manner, the circuit 103 drives the base of transistor 47 incircuit 105 with a current corresponding to the absolute value of theinput control signal applied to the terminals P and O, and determinesthe phase of the timing signal E by the potentials on lines j and k inresponse to the polarily of the input signal.

The circuit 105 drives the windings 101 of the motor with a currentaccording to the current supplied to the base of transistor 47 and witha timing signal E obtained from the timing signal generator 104.

Transistor 47 amplifies the base current supplied from the circuit 103.Transistors 55, 56 and 57, having common emitters, form a three-inputdifferential switching circuit in which the transistors are turned onsequentially in accordance with the timing signals on lines g, h and i,respectively, to drive corresponding transistors 58, 59 and 60. A lowbase voltage initiates the conduction of transistors 55-57. Thecollector current magnitudes of these transistors are determined by themagnitude of the driving current applied to the base of transistor 47.

Transistors 61, 62 and 63 have their emitters connected to a resistor64, and also form a three-input differential switching circuit. However,in this switching circuit the transistor which has the highest basepotential is turned on. The transistors are turned on sequentially inaccordance with the timing signals on lines g, h and i, respectively, todrive the corresponding transistors 65, 66 and 67.

When the potential on line k is higher than that on line j at a time t₁(FIG, 4), transistors 60 and 66 are turned on and current flows throughthe emitter-collector of transistor 66, the Y connected windings 68, 69,and the collector-emitter of transistor 60, in that order.

On the other hand, when the potential on line j is higher than that online k, transistors 67 and 59 are turned on and current flows throughthe emitter-collector transistor 67, windings 69 and 68, and thecollector-emitter of transistor 59, in that order. Thus, the currentflow is reversed with respect to the former case and the result motortorque is therefore also reversed.

The current flowing through the stator windings is detected by aresistor 71 to thereby feed back a portion of such current to theresistors 72 and 73 in the circuit 103. By reason of such feedback thelinearity of the stator winding drive currents with respect to the inputcontrol signal A is improved and variations in the driving transistorcharacteristics are compensated for, resulting in more uniform drivecurrents for each stator phase.

As developed above, even when the stator and the rotor have the samerelative position, the direction of the motor torque is still reversedin accordance with the polarity of the input control signal. Thus, it ispossible to obtain a brushless d.c. motor having the linear torquecharacteristics shown in FIG. 2 irrespective of the polarity of theinput control signal, and such a motor is capable of high speed control,high speed stopping, and high acceleration with relative ease. Suchcharacteristics are particularly, although by no means exclusively,desirable in connection with a magnetic recording tape deck.

What is claimed is:
 1. A brushless d.c. motor, comprising:a. a statorincluding multi-phase windings, b. a rotor having a plurality ofmagnetic poles, c. position detecting means for producing output signalsin response to the positional relationship between said stator and saidrotor, d. means including a comparator for producing a first outputsignal in response to the absolute value of an input control signal anda second output signal in response to the polarity of said input controlsignal, e. means for producing a plurality of timing signals having aphase sequence in accordance with the polarity of said second outputsignal and timed in accordance with the output signals from saidposition detection means, and f. means for sequentially driving saidmulti-phase stator windings with currents having magnitudes proportionalto the absolute value of said first output signal and a phase sequencecorresponding to that of said timing signals, whereby reversible torquesmay be produced in said motor.
 2. A brushless d.c. motor as defined inclaim 1, wherein said means including a comparator comprises a pair ofdifferential amplifiers.
 3. A brushless d.c. motor as defined in claim1, wherein said position detecting means comprises a plurality of Hallelements mounted on a stator assembly and spaced around the periphery ofsaid rotor.
 4. A brushless d.c. motor as defined in claim 2, whereinsaid position detecting means comprises a plurality of Hall elementsmounted on a stator assembly and spaced around the periphery of saidrotor.
 5. A brushless d.c. motor as defined in claim 1, wherein saidtiming signal producing means comprises three pairs of differentialamplifiers.
 6. A brushless d.c. motor as defined in claim 3, whereinsaid timing signal producing means comprises three pairs of differentialamplifiers.
 7. A brushless d.c. motor as defined in claim 4, whereinsaid timing signal producing means comprises three pairs of differentialamplifiers.